(1) Field of the Invention
This invention relates to methods used to create semiconductor memory devices, and more specifically to a method used to create a capacitor structure, for a dynamic random access memory, (DRAM), device.
(2) Description of Prior Art
The semiconductor industry is continually striving to increase the density of DRAM chips, while still attempting to maintain, or increase the performance of the DRAM devices. The decreasing dimensions used to obtain high density DRAM devices, have resulted in difficulties in maintaining, or increasing the capacitance, for the high density DRAM devices. The capacitance of a stacked capacitor structure is positively influenced by decreasing the thickness of the dielectric layer, residing between conductive capacitor plates. Capacitance may also be increased by increasing the area of the capacitor. However decreasing the already thin dielectric layers, now used for DRAM capacitors, can lead to yield and reliability concerns, while stacked capacitor structures, formed in an area overlying transfer gate transistors, are now limited in dimension by the shrinking size of the underlying transfer gate transistor, thus limiting available capacitor area.
An approach used to increase stacked capacitor area, without increasing the length and width of the storage node electrode, of the stacked capacitor structure, has been the use of a roughened surface, storage node electrode. The roughened surface, storage node electrode, comprised of concave and convex features, results in an increase in storage node electrode surface area, when compared to flat surface, storage node electrode counterparts. One method used to create roughened surface, storage node electrodes, has been the use of hemi-spherical grain (HSG), silicon for the upper layer of the storage node electrode. Lou, et al, in U.S. Pat. No. 5,597,754, describe a process for forming an HSG, silicon layer on an underlying polysilicon layer, and patterning these layers to form the increased surface area, storage node electrode, for the capacitor structure of DRAM device. However the HSG silicon solution can be difficult to control, since the amount of HSG silicon roughness is obtainable only in a narrow range of deposition conditions, such as temperature and pressure. This invention will show an alternative to the HSG silicon solution, in which the surface roughness, of the storage node electrode, is established using a composite layer of titanium disilicide, on a polysilicon layer. The desired roughness is accomplished via specific heat treatment processes, resulting in agglomeration of the titanium disilicide layer, producing the desired roughened surface. Prior art, such as Armacost et al, in U.S. Pat. No. 5,466,626, suggest the use of agglomerated titanium disilicide as an etch mask, to create crevices, or a roughened surface, in an underlying polysilicon layer, with the titanium disilicide layer, subsequently removed. This invention however will use the agglomerated layer as a critical component of the composite layered, storage node electrode.
It is an object of this invention to increase the surface area of a storage node electrode of a capacitor structure, used for a DRAM device.
It is another object of this invention to form a storage node electrode-comprised of a composite layer of metal suicide on a polysilicon layer.
It is still another object of this invention to increase the surface area of the storage node electrode by converting a smooth surface of the metal silicide layer, to a roughened surface, via an agglomeration process.
In accordance with the present invention a process is described for forming a composite layered, storage node electrode, for a DRAM device, in which the surface area of the storage node electrode is increased as a result of the use of an agglomerated metal silicide layer, used as the upper layer of the composite layered, storage node electrode. An underlying transfer gate transistor is provided with a storage node contact hole, opened in an insulator layer, exposing a source and drain region of the underlying transfer gate transistor. A conductive contact plug is formed in the storage node contact hole followed by the deposition of a first polysilicon layer, and of an overlying insulator layer. A mesa shape, comprised of the insulator layer, and of the first polysilicon layer, is formed on the underlying composite insulator layer, and contacting the top surface of the conductive contact plug, in the storage node contact hole. A second polysilicon layer is deposited and subjected to an anisotropic reactive ion etch, (RIE), procedure, resulting in second polysilicon layer spacers, formed on the sides of the mesa shape. Removal of the insulator layer, exposed in the mesa shape, results in a polysilicon cylindrical profile, comprised of a bottom polysilicon shape, formed from the first polysilicon layer, and vertical polysilicon shapes, formed from the second polysilicon layer. A metal layer is next deposited, and annealed to convert the metal layer, to a metal silicide layer, in regions in which the metal layer interfaced the polysilicon cylindrical profile, followed by selective removal of the unreacted metal layer. A second anneal is used to convert the smooth surfaced, metal silicide layer, on the underlying polysilicon cylindrical profile, to a roughened surface, metal silicide layer, resulting in a storage node electrode, with increased surface area. The formation of a dielectric layer, on the roughened surface metal silicide layer, followed by the formation of an overlying polysilicon electrode, complete the fabrication of the DRAM capacitor structure, featuring increased surface area, as a result of a roughened surface, storage node electrode.